1. Field of the Invention
The present invention relates to a hydraulic pressure mechanism for supplying working oil to and discharging working oil from a tilt cylinder to angularly lift and lower an outboard engine at the stern of a marine vessel.
2. Description of the Relevant Art
Marine propulsion units for use on small marine vessels such as motorboats comprise an outboard engine, a stern bracket fixedly mounted on the transom of the motorboat hull, and a swivel bracket pivotally supported on the stern bracket, the outboard engine being mounted on the swivel bracket. The swivel bracket is vertically swung with respect to the stern bracket by a tilt cylinder unit, so that the underwater angle of the outboard unit can be adjusted and the outboard engine can be lifted out of water.
One conventional hydraulic pressure mechanism for supplying working oil to and discharging working oil from such a tilt cylinder unit is shown in FIG. 3 of the accompanying drawings. In FIG. 3, the hydraulic pressure mechanism is shown as being in a neutral position. To extend a tilt cylinder unit 103 from the neutral position, a pump 100 is rotated in a normal direction to supply working oil to a righthand port P1 of a spool valve 101 to develop a pressure buildup which opens a check valve 102. Now, the working oil is supplied from the check valve 102 to a lower oil chamber S2 in the tilt cylinder 103, thus moving a piston 104 thereof to the right. At this time, a valve body 105 of the spool valve 101 is moved to the left by the pressure buildup applied to the righthand port P1, causing a lefthand projection on the valve body 105 to push open a check valve 106. Therefore, working oil in an upper oil chamber S1 in the tilt cylinder unit 103 flows through a lefthand port P2 of the spool valve 101 back to the pump 100.
To contract the tilt cylinder unit 103, the pump 100 is reversed to supply and discharge the working oil in the opposite directions to those described above. The tilt cylinder unit 103 has a free piston 111, a damping force generating valve 112, and a check valve 113.
After the lower oil chamber S2 in the tilt cylinder 103 is supplied and filled up with working oil and the piston 104 reaches the upper limit in the upper oil chamber S1 by normal rotation of the pump 100, excessive working oil from the pump 100 returns to a tank 107 through a valve 109 (up-blow valve). After the upper oil chamber S1 in the tilt cylinder unit 103 is supplied and filled up with working oil by reverse rotation of the pump 100, excessive working oil from the pump 100 returns to the tank 107 through a valve 108 (down-blow valve).
When the marine vessel is landed, the tilt cylinder unit 103 may be left extended. At this time, the working oil which is filled in the oil chamber S2 is expanded with the heat of sunlight or the like. When the pressure of the expanded working oil exceeds a preset release pressure level of a valve 110 (thermal-blow valve), the valve 110 is opened allowing the working oil to flow therethrough back to the tank 107. Therefore, the cylinder unit 103 is prevented from being burst.
With the conventional hydraulic circuit which has the valve 110, a pressure buildup higher than the release pressure level of the valve 110 may be developed by propulsive forces produced by the outboard engine, rather than the heat of sunlight. For example, such high propulsive forces may be produced when the ship runs in the shallows. At this time, the piston 104 of the cylinder unit 103 may not be kept in the middle position which allows the ship to run in the shallows, and the ship may not be able to run in the shallows. To avoid this, a stopper or the like is mounted on the stern bracket or the like for mechanically preventing the outboard engine from swinging down beyond a certain angle.